Point mutations in cauliflower mosaic virus gene VI confer host-specific symptom changes.

Mutants of cauliflower mosaic virus (CaMV) strain D4 have been characterized with regard to host-specific phenotypes that resulted from specific changes in the viral DNA sequence. Both the mutant and the wild-type viruses infect a brassicaceous host, Brassica campestris, systemically, giving indistinguishable symptoms. However, in the solanaceous host Datura stramonium, which was systemically infectible by the wild-type virus, mutants induced necrotic local lesions at 21 degrees C and above, and a veinal necrosis at lower temperatures. The mutants differed from the parental D4 strain by having single base changes in gene VI. The necrotic phenotype could be selected during serial passage of D4 in B. campestris or created by site-directed mutagenesis within the gene VI coding region. Full-length 62-kDa gene VI gene products were detected in extracts of plants infected with the mutant strains, as were two smaller proteins derived from the same coding region. The relationship of the host-specific phenotypes of the mutants to the detected gene VI-encoded proteins is discussed in the context of the natural variation found in this gene.